1. Field of the Invention
The present invention relates in general to a driving device and its driving method for a liquid crystal display (LCD) panel. In particular, the present invention relates to a driving circuit and a driving method for rapidly switching the gray levels of a pixel.
2. Description of the Related Art
While there are several types of liquid crystal displays (LCDs), all LCDs operate on the same general principle. A liquid crystal material is placed in a sealed but light transmissive chamber and light transmissive electrodes are placed above and below the liquid crystal material. One type of LCD utilizes twisted nematic liquid crystals, when sufficient electric potential is applied between the electrodes, the liquid crystal molecules change their alignment. The change in alignment alters the polarization state of light passing through the liquid crystal material. The chamber or cell essentially acts as a light shutter or valve, letting a maximum, minimum or some intermediate level of light go through. These levels of light transmittance are called gray levels.
A matrix LCD structure is normally utilized for complex displays. A large number of very small independent regions of liquid crystal material are positioned in a plane. Each of these regions is generally called a picture element or pixel. These pixels are usually arranged in rows and columns forming a matrix. Corresponding numbers of column and row electrodes are correlated with the rows and columns of pixels. An electric potential, also called a driving force, can therefore be applied to any pixel by selection of appropriate row and column electrodes, then a desired graphic can be generated.
The amplitude of a driving force for a pixel depends solely on the gray level that the pixel is going to present. FIG. 1 is a relationship diagram between the light transmittance of a liquid crystal material and the driving voltage. Digitized by 4 bits, for example, the light transmittance is represented by 16 gray levels, Gon to Goff. The oblique line in FIG. 1, enables determination of the driving forces, Von to Voff, for driving the liquid crystal material to respectively display the gray levels Gon to Goff under a static condition. The conventional method for driving a pixel is to provide a driving force without consideration of dynamic switching. That is, if a pixel driver consecutively receives signals of gray level in a sequence of [G2, Gon, G4, G5], for example, it consecutively provides the respective static driving voltages in a sequence of [V2, Von, V4, V5] to the pixel.
However, in real situation, the pixel does not necessarily have the gray level corresponding to the driving voltage shown in FIG. 1, particularly when the driving voltage of the pixel changes in a narrow range of variation. In fact, the smaller the difference in the driving voltage is, the poorer the response rate of the pixel is. In other words, the gray level switching between all-black and all-white is faster than the switching between intermediate levels. Thus, the pixel is not able to display the current gray level.